This invention relates to improvements in an electron gun structure for a color picture tube apparatus.
In general, color picture tubes with three electron gun systems are currently used. Particularly, a color picture tube with an in-line type electron gun is currently used, since self-convergence of three electron beams is easily achieved by using non-uniform deflection magnetic fields for deflecting three electron beams 1, 2 and 3. These fields consist of pincushion type horizontal deflection magnetic field shown in FIG. 1A and a barrel type vertical deflection magnetic field shown in FIG. 1B. Further, it is possible to reduce the power consumption in a color picture tube of the self-convergence type and it is also possible to improve quality and performance, because of its simple construction.
On the other hand, the color picture tube has the disadvantage that the resolution at the periphery of the screen is reduced due to such a non-uniform deflection magnetic field. Namely, the shape of the electron beam on the screen is distorted in accordance with the deflection angle of the electron beam. As shown in FIG. 2, the beam spot 4 in the center of the screen is almost circular, but the beam spot 5 at the periphery of the screen is distorted, so that the electron beam consists of horizontally elongated elliptical shape core 6 with a high brightness and a vertically elongated halo 7 with low brightness. Consequently, the resolution at the periphery of the screen is greatly reduced.
Such beam distortion, because of the non-uniform deflection magnetic field shown is FIG. 2, is caused by the mechanism that the focus of the electron beam in the deflection magnetic field is weakened in the horizontal direction, while the focus in the vertical direction is strengthened. Accordingly, the electron beam at the periphery of the screen is deformed.
The reduction of the resolution due to such beam distortion can be reduced in some degree by suppressing the diameter of the electron beam which passes through the main lens and the deflection region. For this purpose, generally, the electron beam may be prefocused by a prefocus lens. However, in this design, there is disadvantage that the beam spot size at the center of the screen is increased, since the crossover diameter increases.
As another design for compensation of such beam distortion, it has been proposed to use an asymmetric lens (astigmatic lens) as the prefocus lens. For example, U.S. Pat. No. 4,443,736, issued to Chen on Apr. 17, 1984, which describes an improved screen grid structure including a first portion having a circular aperture, a second portion having at least one elongated aperture and a third portion having a circular aperture. Since the electron beam is in the condition of under-focused in the vertical direction by the asymmetric lens, such deflection distortion can be reduced. In this design, however, the beam spot at the center of the screen becames elliptical with the long axis in the vertical direction, so that resolution at the center of the screen is reduced.
As another design for compensation of the beam distortion, it has also been proposed to use a quadrupole lens. For example, Japanese Laid-Open patent application Nos. 61-39346 and 61-39347 describe first and second pairs of plate electrodes with non-circular openings, which are provided between a first and second focus electrodes. A first focusing voltage is applied to both of the first pair of plate electrodes and the first focusing electrode, and a second focusing voltage is applied to both of the second pair of plate electrodes and second focusing electrode. Thus, the quadrupole lens is formed at the plate electrodes. In addition, at least one of the focusing voltage is varied in accordance with the deflection angle to compensate for the beam distortion through the entire screen.
European patent applications with Publication Nos. 231964 and 235975 also describe an electron gun structure having a quadrupole lens to compensate for the beam distortion.
The former application discloses an electron gun structure for a color picture tube including first and second quadrupole lens electrode between a beam forming region and a main focusing lens region to provide a quadrupole lens. The first and second focusing voltages are applied to the quadrupole lens electrodes, respectively.
The latter application discloses an electron gun structure for a color picture tube having first and second focusing electrodes to provide a main focusing lens between them. The first focusing electrode consists of a pair of cup-shaped electrodes with a plate-shaped supplemental electrode between them. The plate-shaped supplemental electrode has three non-circular openings where the electron beams pass. By applying a control voltage to the supplemental electrode, beam spots with an optimum size are obtained throughout the entire screen, since the quadrupole lens is constructed at the supplemental electrode.
Such electron gun structure having a quadrupole lens separated from the main focusing lens may obtain improved resolution over screen center and screen periphery in some degree in comparison with the electron gun structure having an asymmetric lens has the prefocus lens. There are, however, some considerable disadvantages in these electron gun structures. That is, since the action of the quadrupole lens is weakened by the separately provided main focusing lens, the resolution at the periphery of the screen is not sufficiently improved. The quadrupole lens as the action of making the distance of the virtual object point from the main focusing lens differ between the horizontal and vertical directions. At the same time, the spread of the electron beam incident upon the main focusing lens is also made to differ between the horizontal and vertical directions. The relationship between the position of the object point and spread of the electron beam incident on the main focusing lens weakens the action of the quadrupole lens. Consequently, when the focusing voltage is dynamically varied in accordance with the beam deflection, improvement of the resolution at the periphery of the screen (hereafter called sensitivity) can not be satisfactorily achieved.
In particular, since it is necessary to achieve sufficient sensitivity in the case of large current performance and of large and wide deflection angle tubes, the resolution at the periphery of the screen can not be sufficiently improved by an electron gun structure with above mentioned design.
Furthermore, the electron gun structure requires a focusing voltage power source which may supply two values of focusing voltages consisting of a constant focusing voltage to establish the main focusing lens and a variable focusing voltage varying in synchronisation with the beam deflection. In general, since the focusing voltage is as high as 7 kV to 8 kV, it is necessary for a conventional color picture tube to supply the focusing voltage through a socket unit attached to pins mounted on the neck portion of the picture tube. Thus, a color picture tube with the electron gun structure does not have interchangeability with the conventional picture tube, Moreover, a special construction is required to prevent arcing at the socket unit because of high focusing voltage when such two focusing voltages are supplied through the socket unit.